The present invention relates to a semiconductor integrated circuit device comprising a non-volatile memory and a central processing unit More particularly, the invention relates to techniques for providing a single-chip microcomputer, a data processing device, or a microprocessor which includes a flash memory and a central processing unit having a single external power supply.
Microcomputers incorporating a flash memory are known by the designations H8/538F, H8/3048 and H8/3434F, available from Hitachi, Ltd.
Memory cell transistors constituting a flash memory each have a floating gate, a control gate, a source and a drain. As such, each memory cell transistor retains binary information representing a charge injection state of its floating gate For example, electrically charging the floating gate of a memory cell transistor brings a threshold voltage of that memory cell into a high state. When the threshold voltage is raised relative to the control gate, the memory cell prevents a current from flowing. Electrically discharging the floating gate of the memory cell lowers the threshold voltage with respect to the control gate, which allows the current to flow through the memory cell. Illustratively, bringing the threshold voltage of the memory cell higher than a word line selecting voltage level of a read state is called an erasure operation (providing a logical “1” which signifies an erasure state); while, bringing the threshold voltage of the memory cell lower than the word line selecting voltage level of the read state is called a programming operation (providing a logical “0” which signifies a programming state). Alternatively, the erasure state and the programming state may be defined inversely in terms of threshold voltage.
Writing or erasing data to or from memory cell transistors presupposes that their floating gates are placed in a high electric field as needed. This requires that the voltage for erasure or programming purposes be higher than the common power supply voltage, such as 3 V or 5 V. Such a high voltage is provided as an external power supply.